1. Field of the Invention
The present invention relates to a TTL automatic flash control device for cameras which is designed to control the intensity of light which is flashed by a flash light source on the basis of the intensity of light from a subject which is made incident through a taking lens when a picture is taken using a flash light.
2. Related Background Art
TTL automatic flash control devices for camera, which are designed to control the intensity of light flashed by a flash light source when pictures are taken using a flash light are known.
The aforementioned type of device has a single light-receiving element for receiving light reflected from the entire surface or a major portion of a film surface. The above device controls the intensity of light flashed by a flash light source on the basis of the intensity of light from a subject which is detected by the light-receiving element when pictures are taken using a flash light. More specifically, flashing of the flash light source is initiated after a shutter is fully opened, and the light from the subject which is reflected by the film surface is received by the light-receiving element. When an integrated value of the intensity of light received reaches a predetermined value, flashing of the flash light source is suspended and the intensity of light which is flashed from the flash light source is thereby controlled so that a picture can be taken using a flash light at a predetermined exposure value.
However, in the conventional device of the aforementioned type, since the light reflected by the entire surface or the major portion of a film is received by the single light-receiving element, flashing is controlled on the basis of the mean intensity of light of an entire field in the former type or on the basis of the intensity of light of the major portion of the field in the latter type. Hence, detailed states of the field cannot be taken into account when flashing control is performed. For example, an adequate exposure of the major portion of the subject on which the photographer desires to take a picture may not be able to be obtained.
In recent years, technologies for overcoming such a problem of the conventional device have been developed. For example, the present applicant has proposed a multiple pattern metering type TTL automatic flash control device (U.S. Ser. No. 560,745).
In the above device, the light reflected by the film surface is received by a photoelectric converter made up of a plurality of light-receiving elements for respectively metering a plurality of areas of a field, and the quantity of light which is flashed by the flash light source is controlled. More specifically, the flash light source is preliminarily flashed immediately before a focal-plane shutter is opened. During the flashing, the rays of light from the subject are reflected by the surface of a shutter curtain, and those reflected rays of light are received by the photoelectric converter. Thereafter, distribution of the field reflected light is detected by separately integrating the outputs of the light-receiving elements. The field reflected light distribution data is processed according to a predetermined multiple pattern algorithm to determine the weighting value of each of the photometric areas which ensures the optimum exposure value of the major subject. Next, main flashing of the flash light source is performed immediately after the shutter is opened, and the light reflected by the surface of the film is received by the photoelectric converter. The output of each of the light-receiving elements of the photoelectric converter is weighted according to the weighting value thereof, and the sum of the weighted values is integrated. When the integrated value reaches a predetermined value, flashing of the flash light source is suspended, and main flashing is thereby completed. Hereinafter, such a flashing control method is referred to as a TTL multiple flashing control.
Flashing energy for the flash light source is stored in a main capacitor incorporated in the flashing control device. There is a limitation to the external dimensions of the general flash light source, and hence the flash light source cannot incorporate a main capacitor having a large capacity which enables it to flash a few times in a very short period of time.
However, in the aforementioned TTL multiple flashing control device, since preliminary flashing is conducted prior to the flashing photography which uses the main flashing, when flashing photographic operations are conducted sequentially, charging energy of the main capacitor may become in short supply and flashing may thus be disabled before the flashing photography is completed.
Particularly, in the bracketing photography in which pictures are taken in sequence on a plurality of frames at gradually changing exposure values, preliminary and main flashings are repeated a number of times corresponding to the number of frames in a short period of time. Hence, at the end of series of frames, a shutter may be released in a disabled flashing state.